1. Field of Invention
This invention relates to a switching power supply unit used for example with computers; and, more particularly, to improvements in such an unit that can start and stop the primary side switching element by start and stop signals generated on the secondary side, or by command from an over-voltage protection circuit for the output voltage located on the secondary side.
2. Description of the Prior Art
In conventional switching power supply units, a plurality of isolators are used to isolate the following signals: a feedback signal for output voltage stabilization; a signal from the over-voltage protection circuit; and a remote control signal that turns "on" and "off" the power using an external switch located on the secondary side. The isolators may comprise photo-couplers or transformers. For example, in Japan UM Application 1-79389 (1989), a single isolator is used to isolate the remote control signals from the over-voltage protection signals.
However, in that prior apparatus, since the isolator for isolating the feedback signal for output voltage stabilization is provided separately, more than one isolator is needed for the entire apparatus, thereby increasing the number and cost of the components. In addition, for such prior apparatus, if isolation is required between the primary side and the secondary side using a photo-coupler or a transformer, the printed wiring board is difficult to design because specified distances and clearances are required by industry safety standards.
In Japan Patent Application 4-156270 (1992), a signal isolator is used to isolate the above three types of signals. However, in that unit since the latch circuit for over-voltage protection is located on the primary side, the remote control signal is not able to release the latch and restart the power supply. In addition, the commercial power supply has to be turned "off" in order to release the latch. Thus, this prior unit cannot be used in some power supply applications.